1. Field of the Invention
The present invention relates to a soldering flux, and more particularly to a flux of low-residual type which leaves substantially no residue after soldering, thereby requiring no subsequent washing process.
2. Description of the Backcround Art
In the washing process of printed-circuit boards after soldering, chlorofluorocarbon solvents (hereinafter referred to as CFCs) are most widely used in the industry at present. Volumetric regulation on the use of CFCs is now being placed on account of environmental problems such as ozone layer destruction, and in the near future, the use of CFCs will be totally restricted. Under these circumstances, the soldering flux of this invention, which leaves substantially no residue after soldering, can serve for eliminating the washing process to comply with the CFCs regulation. Furthermore, it can assure a more perfect detection of poor soldering by the in-circuit testing after soldering without any difficulties.
Conventional soldering fluxes consisting of natural resin such as rosin and the like or synthetic resin in conjunction with a few kinds of hydrohalogenated amine salts or certain organic acids are well known in the art. However, such fluxes inevitably leave a significant amount of residue on the surface of printed-circuit boards after soldering because they contain a large amount of resin as its essential component. Consequently, they cannot be used without subsequent washing process for the in-circuit testing. Further, the use of CFCs as a washing solvent encounters a strict regulation in connection with environmental problems and becomes more difficult to use in the industry.
Soldering fluxes of low-residual type are being sold by several manufacturers and available on the market. Nevertheless, their flux activities are inferior to those of conventional rosin type because those fluxes are subdued in their solderability to avoid the corrosion problem and/or deterioration in electric resistance. In this respect, such kind of fluxes can never be applicable to the recent surface-mount processes. Particularly, sufficient soldering may not be expected on the deteriorated copper surfaces subjected to thermal shock during the reflow soldering or on the solder-coated surface.